Magnetic devices include transformers, inductors and/or the like. A magnetic device typically includes a magnetic core formed of suitable magnetic materials such as ferrite, powder iron and/or the like. The magnetic device may further include a conductive winding or a plurality of conductive windings. The windings and the current flowing through the windings may generate a magnetic field, which is also known as magnetic flux. In a normal design, the magnetic core usually has a relatively high permeability in comparison with the surrounding medium (e.g., air). As a result, the magnetic flux is confined with the magnetic core, which forms a closed flux path. The magnetic flux provides a medium for storing, transferring or releasing electromagnetic energy.
Coupled inductors are widely used in the power electronics industry. A coupled inductor may comprise two windings magnetically coupled to each other. The two coupled windings may be wound on a same magnetic core (e.g., a toroid core). The first winding generates a first magnetic force, which drives a first magnetic field or flux. The flux generated by the first winding is confined with the magnetic core, which forms a closed flux path. Likewise, the second winding generates a second magnetic force, which drives a second magnetic field, which is confined with the magnetic core.
The magnetic material of the magnetic core of a coupled inductor may be of a magnetic permeability greater than that of a surrounding medium (e.g., air). However, the coupling between two windings of the coupled inductor is not perfect. There may be a leakage path between the winding and the surrounding medium having a lower magnetic permeability. The coupling between the winding the surrounding medium may generate leakage magnetic flux. In an equivalent circuit of a coupled inductor, the leakage magnetic flux is replaced by a leakage inductance.